The present invention is directed to systems for maintaining synchronization among separate television cameras. In particular, it concerns systems for keeping the camera outputs in synchronism at a common switching point.
During the course of a television broadcast, video signals may be drawn from various sources in succession, and it is necessary that the synchronization signals from these sources coincide so that switching from one source to another does not result in such a change in timing as to cause receiving sets temporarily to lose synchronization and color phase reference.
At a television station, this synchronism is typically achieved by keeping a record of the last frame or two of the incoming signal and playing it back with a delay that causes its synchronization signals to coincide with those of the signals currently being transmitted. However, although this type of an arrangement is suitable at a television station, it is often impractical when a transmitter at a location remote from the station is being switched among signals from several cameras.
Accordingly, equipment at the remote location includes a device that sends synchronization signals to the several cameras so that the composite video signals from the various cameras are nominally in synchronism. Since there are differing delays in the various lines to and from the television cameras, though, it is necessary to adjust the timing relationship between the signals sent to the different cameras so that the sync signals from those cameras are in fact synchronized at the switching circuitry.
This requirement of synchronism necessitates the presence at the remote location of someone qualified to make the necessary timing adjustments. The timing adjustment is typically performed before a broadcast begins. But a particular set of adjustments is valid only so long as the electrical distances to the several cameras do not change. If it becomes necessary during the broadcast to increase the length of line to a particular camera, a delay adjustment must be made during the broadcast, too, and such an adjustment is often inconvenient at that time.
It is accordingly an object of the present invention to eliminate the need for personnel to make such timing adjustments.